JEWELLERS ARE being urged to register their stocks of freshwater pearls in an attempt to stamp out a thriving black market that threatens to wipe out one of Scotland's most important natural species.

Six years after a ban on the fishing, sale or advertising of freshwater pearls, officers from Scotland's wildlife crime unit in Strathclyde Police have found freshwater pearls advertised in a spot check of jewellery shops in Glasgow.

Pearl poachers have been detected on the River Tay, in Perthshire, the River Dee in Aberdeenshire and, on the west coast, divers have been reported combing the deep river mussel beds. Under the new Nature Conservation (Scotland) 2004 Act, freshwater pearl-fishing can mean a jail term.

Pearl-fishers kill all the muscles they catch but most mussels do not contain pearls. The endangered species used to be found in more than 160 rivers but it is now thought to have been reduced to 61 breeding colonies in about a dozen Scottish waterways. Scotland holds half the world's population of freshwater pearl mussels.

Conservationists fear the increased poaching could force extinction within 25 years. The species is capable of living for more than 100 years and does not usually mature and start reproducing it is 10 years old. The adult mussels spawn on riverbeds and their fertilised eggs are ingested by migrating salmon and sea trout as they return to their home rivers to breed. The mussel larvae attach themselves to the gills of the fish as a parasite before transforming into tiny mussels and falling to the seabed. Of the 210 million spat a female mussel produces in her lifetime, it is thought only two reach maturity.

Although jewellers can apply to the Scottish Executive for a licence to sell pearls acquired before 1998, only two have been issued, yet many jewellery shops across the UK continue to stock them.

Their attraction dates back more than 2,000 years. The Roman historian Suetonius described Julius Caesar's desire for them as being one of the reasons for invading Britain in 55BC. They are also part of the Scottish crown jewels, including the giant Kellie pearl, the largest found in Britain. Alexander I, the 12th-century king of Scotland, was said to have the world's best collection of freshwater pearls.

"They are often marketed as the ideal present because of their beauty and rarity," said John Ralston, the Scottish Natural Heritage (SNH) licensing officer. "But many people are unaware that is actually illegal to sell them except when the jeweller has a licence from the Scottish Executive." He said one way of controlling the market would be to ask jewellers to submit a list every year of what stock they have so conservation experts can gauge what is in public circulation. "At some point, there will be no more pearls to be sold and we will be able to control new ones coming on to the market. We are trying to determine how organised the poaching is. There are travelling folk involved who seem to see it as their birthright and part of their heritage to harvest these pearls as they travel."

A spokeswoman for the National Association of Goldsmiths said they supported the SNH position and agreed their members should know the laws. But she doubted that a voluntary register of existing stocks would do much to control the market.